Multiple types of 3D sensors exist today. Some of them offer very high depth accuracy. They are mostly limited in terms of acquisition speed and measurement distance. They all require that no or very few light pulses are emitted before a previously emitted pulse is received and measured. This method implies a very limited acquisition speed, in particular for distant objects. This limitation arises from the need to avoid confusion between pulses “in the pipeline” as well as from the use of common optics for emission and reception of pulses. The measurement distance is also limited by the low optical power of the pulse they use to maintain eye safety. In addition, they use multiple low-power pulses separated by large time intervals and then average them over a long time in order to measure a 3D pixel. Their data throughput is not substantially enhanced due to the lack of on-chip compression capabilities.
The following methods are usually used for 3D data acquisition.                Triangulation: a triangulation 3D laser scanner is an active scanner that uses laser light to probe the environment. The triangulation laser shines a laser beam on the subject and exploits a camera to register the location of the laser dot. Depending on how far away the laser strikes a surface, the laser dot appears at different places in the camera's field of view. This technique is called triangulation because the distance is calculated based on parameters of the triangle created by the laser dot, the camera and the laser emitter;        Modulated light: modulated light 3D scanners shine light with ever changing optical power on the subject. Usually the light source simply cycles its power in a sinusoidal pattern. A camera detects the reflected light, and the amount the pattern is temporally shifted reveals the distance the light traveled. Modulated light also allows the scanner to ignore light from sources other than the laser, so there is minimal interference;        Structured light: structured-light 3D scanners project a pattern of light on the subject and register the deformation of the pattern as observed in one or several particular directions. The pattern may be one-dimensional or two-dimensional. The simplest example of a one-dimensional pattern is a line. The line is projected onto the subject using either an LCD projector or a sweeping laser beam. A camera, offset slightly from the pattern projector, registers the shape of the line and uses a technique similar to triangulation to calculate the distance of every point on the line. In the case of a single-line pattern, the line is swept across the field of view to gather distance information one strip at a time;        Time of flight: a time-of-flight 3D laser scanner is an active scanner that uses laser light to probe the subject. It measures the distance of a target point via the round-trip time of a pulse of light. A laser is used to emit a short light pulse, and a detector registers the arrival of the reflected light. Since the speed of light c is known, the round-trip time determines the travel distance of the light, which is twice the distance between the scanner and the surface. If t is the round-trip time, then distance is equal to c*t/2. The accuracy of a time-of-flight 3D laser scanner depends on how precisely it can measure the time delay t: 3.3 picoseconds (approx.) is the time taken for light to travel over one millimeter.        
The most advanced scanning systems proposed until now (Example, Source: http://www.breuckmann.com/en/bodymetrie-life-science/products/product-overview.html) do not have the capability to make very rapid 3D measurements of a scene with high depth precision. They cannot operate at long distances (more than 10 meters) and at the same time at high speed, as required, for example, to accurately image human persons walking at a normal speed. Their use would thus require that humans stop moving in order to acquire 3D data concerning their shape and position.
A paper entitled “Three Dimensional Flash Ladar Focal Planes and Time dependant Imaging-ISSSR Paper” by R. Stettner, H. Bailey, and S. Silverman describes a method of producing an image on the basis of sending flashes of laser light onto a scene. A focal plane array acquires an entire frame at each laser flash. The distance information of a dot in the scene is evaluated on the basis of the time of flight of the laser flash when it is detected at the 2D focal plane array detector. The process described in that paper appears to use APD and not SPAD, i.e., it uses analog time-of-flight measurements. Pulses with high energy pulses at 1.5 μm and a low repetition rate of 30 Hz are used. The pulse duration is not specified but is probably much longer than 1 picosecond. No use of a diffuser is described. The link between the sensors per-se and the data treatment chips is complex and difficult to industrialize.
The present invention is exemplarily based on the use of ultra-sensitive SPADs, which can be easily fabricated with a CMOS process, combined with a simple low cost germanium process. A large number of pulses are emitted toward each point on the object. This creates the advantage of a large reduction of the uncertainty of the 3D pixel position linked to the SPAD's inherent timing jitter (around 30 to 90 picoseconds for a single measurement). The duration of the pulse is preferentially chosen to be lower than the expected final uncertainty on the 3D depth, so that the pulse duration is not limiting the measurement accuracy. The required light pulses can be generated with a mode-locked laser, possibly followed by an erbium amplifier. for increasing the pulse energy.
Another paper entitled “Geiger-mode avalanche photodiode focal plane arrays for three-dimensional imaging LADAR” by Mark A. Itzler and all. also describes a method of producing an image on the basis of flashes of laser light impinging a scene. The process described in that paper uses SPAD arrays operating at 1.1 or 1.5 μm. It is essentially a single pulse (per measurement) process with time measurement based on a fast counter: “An overall 13-bit timing resolution is obtained using 11-bit pseudorandom counters with two additional Vernier bits created by using a copy of the clock with a 90 degree phase shift”. No concept of using pulse trains or data compression is involved.
The typical characteristics of a commercial high quality 3D scanner (FARO LS 880 HE80) include a maximum measurement rate of 120,000 pixels/s, a laser wavelength of 785 nm, a vertical field of view 320°, a horizontal field of scan view 360°, and a linearity error of 3 mm (at 25 m and 84% reflectivity).
In some situations, the fastest 3D sensors are in fact 2D sensors (ultrafast video cameras) working in tandem in a stereoscopic arrangement in order to provide the depth information. The data throughput of the existing 3D sensors is limited by the bandwidth of the system links to the “external” world and the effective use of DSP for 2D/3D data treatment.
Very few 3D sensors are using 1.5 μm near-infrared (NIR) wavelength. These are mainly long distance rangefinders or airborne Lidar using the relatively eye-safe properties of this wavelength range (due to the absorption of the IR light in the eye's lens) to operate at very long distances (The low absorption of the atmosphere even in cloudy weather is another advantage.) They generally do not use erbium amplifiers (with their light energy storage capacity, allowing for high peak powers). This spectral region is mainly used by the telecommunication industry for fiber-optic data transmission.
Single-photon avalanche diodes (SPADs) are new detectors which are able to switch (avalanche) when receiving single photons. In the state of the art, they are implemented on silicon, and are mainly sensitive in the spectral range with wavelengths from 0.4 μm to 0.9 μm. The maximum frequency of the pulses measured by a SPAD detector is limited by its large recovery time (around 20 to 100 ns). The timing precision is limited by the intrinsic jitter, which is around 50 to 200 ps.
The development of SPAD for replacing bulky and costly photomultiplier tubes and for producing a large matrix of ultrasensitive detectors has been an ongoing process from the 1980s when S. Cova et al. developed these new silicon devices.
Recently, the MEGAFRAME European consortium was created to design SPADs with complex ancillary electronics at the pixel level. The components for one pixel comprise a SPAD and electronics for gating functionality, a time-uncorrelated photon-counting facility, and chronometer single-photon counting. The word Megaframe refers to the ambition of the project to capture more than one million images (frames) per second. An example on such a realization can be found at: http://www.megaframe.eu/Contents/Publications/MEGAFRAME/FET09_MF2_poster_black_v2.pdf
Two recent reviews describe the state of the art in SPAD development: SPAD Sensors Come of Age, Edoardo Charbon and Silvano Donati. Optics and Photonics News, Vol. 21, Issue 2, pp. 34-41 (2010); and also A low-noise single-photon detector implemented in a 130 nm CMOS imaging process. Marek Gersbach, Justin Richardson, Eric Mazaleyrat, Stephane Hardillier, Cristiano Niclass, Robert Henderson, Lindsay Grant, Edoardo Charbon. Solid-State Electronics 53 (2009) 803-808.
The SPAD detectors of prior art suffered from important drawbacks. The largest matrices of SPADs detectors available today are 128 by 128 detectors due to a tradeoff between the size of the chip and the requirement of the optic which is mounted in front the SPADs detector. To reach an optimal 3D dept resolution, results of multiple time-of-flight measurements should be averaged on each SPAD detector
Each SPAD diode must be connected to a time-to-digital converter (TDC). In most prior art, such TDC devices are multiplexed between the SPAD detectors. That feature does not allow high speed imaging because the SPAD detector must wait until all of the SPAD diodes have finished their multiplexed detection at the few available TDCs.
Further, in the prior art, the pulse frequency has an upper limit which is the inverse of the time of flight between the emitter, the target and the detector. This limits the measurement speed, and that limitation may prevent the measurement of moving objects in a scene at a high repetition rate as it is required for a real-time imaging service with acceptable depth resolution.
Further, in the prior art, all detection signals are transmitted to an external specialized processor, such as a digital signal processor, which is separate from the SPAD detector integrated circuit. Therefore, the throughput of the data transferred is constrained by the limited bandwidth of external (chip-to-chip) links. Internal chip links can have a much larger bandwidth and could be built in large quantities on small surface with few limitations. Existing SPAD matrix devices do not use local averaging between adjacent detectors to optimize the tradeoff between speed and sensitivity.
It appears that the state of the art of SPAD sensors is not sufficient for obtaining a representation of a scene with moving persons, for example. The frame rate would be so slow that the movement of persons with typical velocities within one frame capture period would be well above the desirable depth resolution. Also, in following the teachings of the state of the art, if the resolution of a representation, as a 2D image, is enhanced, the quantity of processed data to provide significant improvement on the clarity of an image exceeds the technical possibilities of the external (non-detector-embedded) known digital signal processing techniques. On the other hand, the speed of the process of acquiring a 2D scene, and more so, a 3D scene, in which an object is moving, is also out of the scope of the existing (non-detector-embedded) techniques.